Traditionally, blood pressure monitoring at home has only required that the results of blood pressure measurements be managed as part of daily health management, and has not required strict measurement results.
However, in recent years, the importance of daily health management has become important, and recent reports recommend that blood pressure be measured a plurality of times under 24-hour ambulatory conditions. By measuring blood pressure a plurality of times for a period of 24 hours, it is possible to monitor variations in blood pressure that could not be found by measuring blood pressure at an appropriate time within a day as would usually be done at home. That is, the basic idea behind the ambulatory blood pressure monitoring is that, by measuring blood pressure a plurality of times rather than only once, it is easier to discover a disease, etc., associated with variations in blood pressure.
For example, blood pressure is normally lower at night or during sleep, but it is reported that there are some hypertensive patients whose blood pressure does not fall even at night or during sleep. On the other hand, there are some whose blood pressure rises at night, which are prone to cerebral infarction, myocardial infarction, or the like, because high pressure continues to be exerted on the blood vessels during the night or during sleep when the pressure exerted on the blood vessels should normally fall. By monitoring variations in blood pressure within a day, it may is possible to detect not only hypertension but other diseases that are said to be related to hypertension.
Recently, terms such as masked hypertension, workplace hypertension, and white coat hypertension have come to be known and used widely. Masked hypertension is a phenomenon in which the blood pressure measured on a person is normal when actually the person is hypertensive.
Workplace hypertension is a form of masked hypertension. That is, when actually a person is hypertensive because the person is physically tired or mentally stressed due to work, the blood pressure measured at home, in a hospital, or in a workplace medical examination lies within the normal blood pressure range because the person is released from the fatigue or stress causes in such an environment. White coat hypertension is a phenomenon in which the blood pressure measured at home is normal but the blood pressure measured in a hospital exhibits a high blood pressure value.
Since white coat hypertension is induced by exposure to a hospital environment or at the sight of a white coat, it is said that the patient can recognize the symptom relatively easily. However, masked hypertension and workplace hypertension present the problem that not only the patient but also clinicians cannot easily recognize the hypertension because the hypertension is not reflected in the actually measured blood pressure values.
Such symptoms that are not easily recognizable even by the patient himself become easier to discover by monitoring variations in blood pressure within a day.
Generally, it is preferable that the blood pressure is measured with the patient in a relaxed state. If the patient is tense, the blood vessels contract and the blood pressure cannot be measured correctly. In view of this, it is often recommended that the patient breathe deeply or stay in a relaxed state for a few minutes before taking the blood pressure. It is therefore desirable, as a general rule, that the patient's physical and mental states at the time of blood pressure measurement be recorded together with the measured blood pressure values.
However, it is difficult to correctly measure and record the patient's physical and mental states. In view of this, some blood pressure monitors are designed to be able to measure pulse rate simultaneously with the blood pressure. By also measuring pulse rate, the patient's physical and mental states can be deduced from the recorded pulse rate. For example, if the pulse rate is high, it is deduced that the patient is under stress.
Further, when measuring the blood pressure, the positional relationship between the part on which the blood pressure is measured and the heart is also important. If the part on which the blood pressure is measured is not positioned correctly, the condition of the blood vessels in that part changes, resulting in an inability to correctly measure the blood pressure. For example, if the position of the part to be measured is lower than the position of the heart, the blood pressure may be measured higher than it really is. More specifically, it is said that the blood pressure rises by 7 mmHg as the position of the part to be measured becomes lower than the position of the heart by 10 cm.
When measuring the blood pressure, the part to be measured must be positioned at the same height as the heart. When a blood pressure measuring cuff is worn on the part to be measured, the position at which the cuff is at the same height as the heart is the correct position, and it is important to measure the blood pressure with the cuff held in the correct position. The correct position implies that the position of the part to be measured relative to the position of the heart is within the correct height range. The correct height range means that the part to be measured is positioned at approximately the same height as the heart, and does not mean that the former should be positioned strictly and exactly at the same height as the latter.
Recently, it has been recommended that the cuff be worn around the upper arm. This is because, first of all, the cuff or the like is easily worn around the upper arm, and also because the part to be measured can be easily held at the same height as the heart even when the patient lies on his back on the bed.
On the other hand, in the field of digital blood pressure monitors, a wrist-worn blood pressure monitor is known that measures blood pressure with the blood pressure monitor itself or the blood pressure measuring part worn on a wrist. Because of its structure, the wrist-worn blood pressure monitor can be made compact in size so that it is convenient to carry and suitable for blood pressure measurement in various environments such as home, workplace, etc. However, the wrist-worn blood pressure monitor requires that the part to be measured be positioned at the same height as the heart. In view of this, it is known to provide a blood pressure monitor in which a sensor is mounted in the blood pressure monitor itself or in the blood pressure monitoring part to detect the position of the part to be measured. For example, a wrist-worn blood pressure monitor is proposed that is equipped with an attitude detecting means constructed from a tilt sensor or the like (patent document 1).
The blood pressure monitor disclosed in patent document 1 will be described with reference to FIG. 10.
FIG. 10(a) is a diagram for explaining the tilting of the part to be measured by the wrist-worn blood pressure monitor disclosed in patent document 1 and the positional relationship of the part to be measured relative to the height position of the heart.
With the wrist-worn blood pressure monitor 112 worn on the wrist 111, the elbow 113 is placed on a desk 114; in this condition, the angle that the elbow 113 makes with the surface of the desk 114 is denoted by θ, and the difference in height between the heart 115 and the wrist-worn blood pressure monitor 112 is denoted by H. The relationship between the tilt θ and the height difference H is a linear function upward to the right, and if the tilt θ can be detected, the height difference H can be determined from the tilt θ.
That is, by detecting the degree of tilting of the wrist-worn blood pressure monitor 112 using a tilt sensor, and thereby obtaining the height position of the wrist-worn blood pressure monitor 112, the subject who is going to measure his blood pressure can hold the wrist-worn blood pressure monitor 112 in the correct position for measurement.
FIG. 10(b) is a diagram for explaining another example of the blood pressure monitor disclosed in patent document 1, illustrating how the position of the part to be measured is aligned with the position of the heart. In the illustrated example, the wrist-worn blood pressure monitor 112 has a built-in heartbeat sensor (more example, a microphone) 116, not a tilt sensor, and the wrist-worn blood pressure monitor 112 is brought close to the front part of the body near the heart to search for the correct position using the heartbeat sensor.
FIG. 10(c) is a graph depicting the strength of the heartbeat sound obtained by the heartbeat sensor 116 in the wrist-worn blood pressure monitor 112 when searching for the correct position as illustrated in FIG. 10(b). The strength of the heartbeat sound is plotted along the ordinate, and the position of the wrist-worn blood pressure monitor 112 is plotted as the scanning position along the abscissa. The wrist-worn blood pressure monitor 112 is pressed against the body of the subject and moved up and down to search the strength of the heartbeat obtained by the heartbeat sensor 116. The position of the heart is detected when the peak value Lp is detected, and the thus detected position is the correct position so far described.
In this way, the blood pressure monitor disclosed in patent document 1 detects the correct position by using the tilt sensor or the heartbeat sensor, after which the subject operates the start switch to start the blood pressure measurement.
Patent document 1: Japanese Patent No. 3297971 (pp. 2-3, FIGS. 2, 7, and 8).